Igneous Rocks
Today: Origins and Classification
Tomorrow: Volcanoes
Thursday: Volcanic Hazards
Mt. Whitney, CA; east face, granodiorite
Pressures and Temperatures of Earth
Temperature (and pressure) increase with depth in the Earth’s
crust. The high temperature is due to heat left over from planetary
accretion (when Earth was formed) and radioactive decay of
elements in the mantle and core.
How Melting Happens
Rocks are solid but under
the high pressures and
temperatures of the crust
at depth, rock melts to
magma. The composition
of the magma determines
what temperature and
pressures will make the
melt into solid crystals
Water lowers the melting temperature
of rock. Water at plate boundaries is a
major factor in generating magma.
Terms
Texture - size, shape, distribution and spatial relationship of
minerals or components of a rock.
Composition - the chemical nature of the minerals or matrix
that makes up the rock.
Matrix - the predominant background material that makes up a
rock; sometimes called groundmass.
El Capitan granodiorite,
medium grained texture
Vein of material in a
fine grained matrix
Uncut diamond in
kimberlite matrix
Classification Schemes
1. Textures - fine grained (aphanitic) or coarse grained
(phaneritic). Porphyritic means both sizes. Special cases phenocrysts, xenocrysts, megacrysts.
2. Composition - minerals, colors; mafic (meaning close to a
basalt; Fe, Mg), intermediate or felsic (rich in felspars, Si, K).
3. Origin of the rock - intrusive, extrusive
4. Relationship to surrounding rocks - what does it intrude?
How was it extruded?
Texture
Intrusive - determined by rate at which the material was cooled,
and relationship to host rock.
Extrusive - determined by process by which the rock was
extruded, as well as interactions with water and gases.
Texture is also indirectly affected by composition.
Sketch of crystals forming in
a magma chamber
Ash plume due to
water and gas
Three
generations of
dikes
intruding
granite.
Composit
ions
range
from
mafic to
felsic.
Composition
Mafic - composed of minerals containing Fe and Mg that form early in
the sequence. Dark colored and less stable on the surface. Example:
basalt (extrusive), and diorite (intrusive).
Intermediate - Both types of mineral, mixed color.
Felsic - composed of minerals containing Si and K (feldspars and
quartz) that form later in the sequence. Light colored and more stable
on the surface. Example: rhyolite (extrusive) and granite (intrusive).
Mafic - diabase;
intrusive
Intermediate - andesite;
extrusive
Felsic - granite;
intrusive
Exception: Obsidian
Obsidian outcrop
Rare banded, translucent obsidian
Obsidian is volcanic glass, and classifies as an extrusive rock.
The molecules are completely disordered meaning they have
no lattice. Obsidian is usually dark but is made of felsic
material, namely SiO2 (up to 70%).
Bowen’s Series
Igneous minerals crystallize in an order with decreasing temperature
Mafic Minerals
Olivine - (MgFe)2SiO4
Rock = peridotite (olivine
and pyroxenes)
Pyroxene - (XY)SiAl2O6
Where XY = Na,Ca, Fe, Mg
Rock = gabbro (pyroxene
and plagioclase
Felsic Minerals
Alkali Feldspar - (X)AlSi3O8
Where X = Ca, Na, K
May make up to as much as
60% of the crust.
Quartz - SiO2; gray, may
only make up no more
than 60% of the rock if it
is an igneous rock.
Extrusive Processes
Plumes - ash, tiny fragments + gas
Flows - magmas, magma + ash + water mixtures
Twin Falls, ID; Columbia river basin flood basalts
Mt. St. Helens, 1980
Instant cooling - crystals do not have time to form.
How was it extruded?
How far did it travel?
Where did it flow or fall (on water or land?)
Is it layered?
Lava / Basalt Examples of different styles:
Pahoehoe - ropey, smooth flowing
A a - chunky, brittle, sounds
like breaking glass as it flows
Vesicles:
bubbles formed
in the rock from
the exsolving of
gas when rock
was still molten.
‘Vesicular’ is a
kind of texture.
Terms
Pumice has small vesicles
and floats in water.
Pryoclastic: a
volcanic rock
that was
transported by
air, water or its
own viscocity.
Clasts can be
tiny or giant.
Tephra - tiny SiO2 ash
fragments; note vesicles
Scoria has larger vesicles and
sinks in water.
Volcanic bomb from Aso volcano,
Japan - note roundness
Intrusive
Depth: a pluton is emplaced deep within the lithosphere
whereas hypabyssal bodies make it to shallow regions
Crystals are Relative
Phaneritic - large crystals with well defined boundaries.
Aphanitic - very small crystals, not necessarily visible.
Porphyritic - both large and small crystals.
Phenocryst - crystal of a different size
Megacryst - huge crystal
Xenocryst - crystal of a different origin
Crystal Textures
Finer grained, approaching aphanitic
Phaneritic with felspar phenocrysts
What
makes a
rock
porphyritic?
Summary
Igneous rocks are classified
according to:
Intrusive or Extrusive
Textures
Compositions
Minerals
Crystal relationships,
boundaries
Relationship to host rock